The present invention is directed to aluminum alloys with optimum combinations of formability, brazeability, corrosion resistance, and hot workability, and methods of use, and in particular, to aluminum alloys having controlled levels of manganese and iron, and a controlled chemistry and levels of intermetallic particles to provide optimum performance in applications such as heat exchangers.
In the prior art, aluminum alloys are the alloys of choice for heat exchanger applications. These alloys are selected for their desirable combination of strength, low weight, good thermal and electrical conductivity, brazeability, optimum corrosion resistance and formability.
Typical applications for heat exchangers include automotive heater cores, radiators, evaporators, condensers, charge air coolers and transmission/engine oil coolers. One particular application that requires a good combination of properties is tubing for radiators, condensers and the like. In these applications, fin stock is arranged between stacked tubing and end sheets that carry the heat transfer media. The tubing is situated between headers which redirect the heat transfer media flow between layers of tubing and which also can contain the heat exchanger inlets and outlets.
In one particular application, the tubing is formed into a u-shape and is threaded through openings in the fin stock and also through openings in end sheets adjacent to the fin stock ends. Once the tubing is inserted, the tubing is internally and diametrically expanded to maximize the metal-to-metal contact with the fin stock and the end sheet, and heat exchange between the tubing and the fin stock.
After the insertion and expansion, the free ends of the tubing extend beyond the fin stock and end sheet for attachment to the header manifold. The length of extension of the tubing beyond the fin stock and end sheet once expanded is critical for subsequent header manifold attachment. This height extending past the end sheet after the expansion process is commonly referred to as a xe2x80x9cstickup height.xe2x80x9d If the length is insufficient for header manifold attachment on just one of the many tubes interleaved in the fin stock, the entire heat exchanger must be rejected. As part of the expansion, the tubing end also becomes bell-shaped with a bell diameter. The measurement of stickup height and the bell diameter gives a good measure of the forming performance and can be used as a standard to determine whether the assembly can be further processed into a complete heat exchanger.
During the expansion step, the tubing will change its dimension, shrinking from its original installed length. This shrinkage can result in a reduction in the stickup height of the free ends of the tubing extending beyond the fin stock and end sheet for header attachment, and rejection of the heat exchanger. Thus, besides the other mechanical properties associated with the aluminum alloys typically used in heat exchanger application, this xe2x80x9cstickup heightxe2x80x9d is crucial and the alloys must exhibit the necessary formability to allow for the expansion without excessive shrinkage and the like.
A current alloy used in these types of applications is AA3102. The Aluminum Association specifies, in weight percent, a compositional makeup for this alloy of up to 0.40% silicon, up to 0.7% iron, up to 0.1% copper, between 0.05 and 0.40% manganese, up to 0.05% zinc, up to 0.03% titanium, with the balance aluminum and inevitable impurities, each impurity up to 0.03%, and total impurities up to 0.10%. This alloy has excellent formability but poor corrosion resistance. Consequently, while the alloy performs ideally in heat exchanger manufacture, the alloy must be coated for corrosion protection.
It is believed that the intermetallic particles found in the matrix of AA3102 contribute to its good formability. FIG. 1 shows a schematic of a micrograph of an AA3102 alloy. The schematic shows a matrix of aluminum designated by the reference numeral 1 and a volume fraction of intermetallic particles 3 distributed throughout in the alloy matrix. This distribution is generally about 3.0% by volume of intermetallics in these prior art alloys. At the same time, the particles 3 are primarily FeAl3, which have an electrolytic potential differing greatly from the aluminum matrix. As explained in more detail below, with the FeAl3 being less negative than the matrix of pure aluminum, the matrix corrodes first under SWAAT conditions. SWAAT corrosion testing uses a well known testing standard, i.e., ASTM G85 Annex 3, and does not need further description for understanding of the invention. Consequently, AA3102 has poor corrosion resistance and must be coated when used in heat exchanger applications.
Other alloys have been developed as disclosed in U.S. Pat. Nos. 5,906,689 and 5,976,278 to Sircar (hereby incorporated in their entirety by reference), which offer high hot workability and improved corrosion resistance. The corrosion resistance of these alloys is so superior to prior art alloys that the need for coating the alloys is eliminated. One reason for this is that the number of intermetallic particles, e.g., FeAl3, that adversely affect corrosion resistance is less.
However, these new alloys lack the intermetallic particle distribution/density that exists in AA3102. As can be seen from FIG. 2, these highly corrosion resistant alloys have a matrix 5 and dispersed intermetallics 7. The schematic of FIG. 2 depicts only about 0.1% volume fraction distribution of the intermetallics 7. As a result of the lower volume fraction of intermetallics 7, these alloys may sometimes lack the needed formability for certain heat exchanger manufacturing operations.
Consequently, a need exists to provide an aluminum alloy composition that combines formability, hot workability and corrosion resistance in one alloy, and an alloy adapted especially for particular use in heat exchanger manufacturing and applications.
Accordingly, it is a first object of the present invention to provide an aluminum alloy having an optimum combination of hot workability, brazeability, corrosion resistance, and formability.
Another object of the present invention is a method of manufacturing the inventive aluminum alloy for use in heat exchanger applications or a method of making the alloy as a sheet or strip rather than tubing for use in other applications wherever the combination of excellent corrosion resistance, brazeability, and formability is desired. Sheet product may also be used to make tubes as found in typical radiators and heater cores.
A still further object of the present invention is a method of manufacturing articles requiring forming the alloys, particularly, expanding the alloys. In particular, the inventive method is directed to improvements in making heat exchangers where the tubing is expanded as part of the assembly process.
Yet another aspect of the invention is the ability to improve formability and provide excellent corrosion resistance in an aluminum alloy without significantly affecting hot workability as compared to conventional alloys and those described in U.S. Pat. Nos. 5,906,689 and 5,976,278 to Sircar.
Other objects and advantages of the present invention will become apparent as a description thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention provides an aluminum alloy article made of an alloy composition comprising, in weight percent:
between about 0.05 and 0.5% silicon;
an amount of iron between about 0.1% and up to 1.0%;
an amount of manganese up to about 2.0%;
between about 0.06 and 1.0% zinc;
between about 0.03 and 0.35% titanium;
with the balance aluminum and inevitable impurities;
wherein the manganese to iron ratio is maintained between greater than about 0.5 and less than or equal to about 6.0, and the iron and manganese amounts total greater than about 0.30%, such that the article contains intermetallic compounds dispersed throughout an aluminum matrix in a volume fraction of the article of at least 0.5%, preferably at least about 2.0%, and wherein a difference in electrolytic potential between an aluminum matrix of the article and the intermetallic compounds is less than about 0.2 volts. The intermetallic compounds can have an aspect ratio of less than about 5.0. The intermetallic compounds can range in size from about 0.5 to 5 microns.
In a preferred embodiment, the ratio of manganese to iron is further limited to a lower limit of 0.75 and an upper limit of about 5.0, more preferably between 1.0 and 4.0, and the manganese and iron total amount is at least about 0.6%, and more preferably between about 0.7 and 1.2%.
The inventive alloy is preferably utilized in extrusion processes that make tubing, particularly, extrusion processes designed to make heat exchanger tubing. The alloy can also be used in sheet form where formability is important.
In another aspect of the invention, the inventive alloy is ideally suited for methods of making heat exchangers that employ an expansion step of the tubing. The alloy composition of the invention, when expanded as part of these processes is superior in terms of formability and providing the requisite stick-up height needed for the manufacturing process. A preferred tubing size is 6 mm in diameter but other sizes can be employed.
The invention also entails a method of improving the corrosion resistance and formability of an aluminum alloy article without loss of hot workability by providing an aluminum alloy composition comprising alloying amounts, in weight percent, of between about 0.05 and 0.5% silicon, an amount of manganese up to about 2.0%, an amount of iron between about 0.1% and up to about 1.0%, between about 0.03 and 0.35% titanium, and between about 0.06 and 1.0% zinc, with the balance aluminum and inevitable impurities, and forming the article, wherein the ratio of manganese to iron in the composition is controlled to between about 0.5 and 6.0, and the total amount of iron and manganese in the composition is controlled to be greater than about 0.3%, so as to form a finished microstructure in the article having greater than about 0.5% volume fraction of intermetallic compounds, the intermetallic compounds having an aspect ratio less than 5.0, and wherein an electrolytic potential difference between an aluminum matrix of the article and the intermetallic compounds is less than about 0.2 volts.